When an optical signal is modulated for transmission through an optical fiber on a carrier frequency fc (fc=C/λ where C is a constant and λ is the optical wavelength), the modulated information signal will have two symmetric sidebands that are centered on the carrier frequency. In order to reduce fading due to fiber dispersion, and to conserve bandwidth in the transmission of such a signal, the optimal solution is to filter out one of the sidebands, either above or below fc. For various technical reasons, however, simultaneous preservation of one complete sideband and a complete removal of the other sideband is impossible. Nevertheless, although one of the sidebands may be partially suppressed, the complete preservation of the other sideband for transmission is highly desirable.
A partial solution for the difficulty mentioned above, is the use of a vestigial sideband (VSB) filter. As is well known in the pertinent art, a VSB filter is a band pass filter that effectively preserves one sideband while partially suppressing the other sideband. Just how much of the unwanted sideband can be actually suppressed, however, is a design consideration. As noted above, it is virtually impossible to suppress 100% of the unwanted sideband. The portion of the sideband which cannot be suppressed is then referred to as the vestigial sideband (VSB).
During a signal transmission it will happen that the VSB, which is transmitted with the unsuppressed sideband, will introduce impairments (distortions) into the transmitted signal. For signal integrity, these impairments need to be avoided, or at least minimized. For example, it is known that telecommunication signals can be adversely affected by group delays (i.e. time delays of amplitude envelopes), and phase delays (i.e. time delays of signal phase). Both of these types of delays result from interferences caused by the VSB. Also, and perhaps of greater concern, are Inter Symbol Interferences (ISI) that are introduced by the VSB during the demodulation of digital signals from an analog carrier signal. In any event, an optical information signal which is transmitted over an optical fiber will be somehow corrupted.
The primary object of VSB control is obviously to minimize impairments (distortions) in the received signal, while also preserving the integrity of the transmitted information signal as much as possible. With this objective in mind, closed loop feedback control technology has provided interesting possibilities.
In the context of signal telecommunications, an overview of closed loop control for a desired system output requires comparing the actual output of a system with the actual system input. In the case of a telecommunications system which seeks to preserve signal integrity, the desired system output will be the same as the system input (i.e. a signal transmission that results in a non-corrupted signal). When they are not the same, the signal has been corrupted during transmission. In this later case, a comparison of the actual output with the actual system input will generate an error signal. In a communications system, where it is known that the transmitted signal will be corrupted, the object is then to minimize the error signal. In essence, the question is what feedback will most effectively minimize the error signal.
An example of employing closed loop technology to control signal transmission using a VSB filter is provided by U.S. Pub. No. 2003/0058509 (hereinafter referred to as “Webb”). As disclosed in Webb, the control loop is used to adjust the wavelength of a laser that is providing the carrier frequency. Alternatively, Webb discloses the use of a wavelength control block to control the filter edge of the VSB filter. Unlike the disclosure of Webb, the present invention incorporates a tapped delay equalizer in the feedback loop which is established to reshape the input signal for the purpose of improving a VSB filter.
In light of the above, it is an object of the present invention to provide a system and method for equalizing a digitally modulated signal, for input as an optical signal to an optical VSB filter, for transmission of the optical signal over an optical fiber. Another object of the present invention is to provide a device which employs a tapped delay equalizer to equalize a digitally modulated signal for subsequent conversion and filtering as an optical signal by a VSB filter for transmission over an optical fiber. Still another object of the present invention is to provide a system for using a tapped delay equalizer, in combination with a VSB filter, to transmit optical signals over an optical fiber which is easy to manufacture, is simple to use and which is comparatively cost effective.